<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      function sum(...args) {
        let res = [...args];
        var _addr = function (...args2) {
          res = [...res, ...args2];
          return _addr;
        };
        _addr.toString = function () {
          return res.reduce((pre, cur) => {
            return pre + cur;
          }, 0);
        };
        return _addr;
      }
      console.log(sum(1)(2)(3).toString()); // 6
      // console.log(sum(1, 2, 3)(4)); // 10
      // console.log(sum(1)(2)(3)(4)(5));
    </script>
  </body>
</html>
